Low induction, high voltage, bushing connections to tanked electrical apparatus



Feb. 25, 1964 T. w. HUNT ETAL 3,122,606

LOW INDUCTION, HIGH VOLTAGE, BUSHING CONNECTIONS TO TANKED ELECTRICAL APPARATUS Filed May 22, 1961 2 Sheets-Sheet 1 4 ruimu fk @066,

Attorney;

Feb. 25, 1964 T. w. HUNT ETAL 3,122,605

LOW INDUCTION, HIGH VOLTAGE, BUSHING CONNECTIONS TO TANKED ELECTRICAL. APPARATUS 2 Sheets-Sheet 2 Filed May 22, 1961 QDOOOOO J5 a/ lcvenior z 3,l22,6tl6 LQW INDUfiTION, HlGH VOLTAGE, BUSHING QONNECTIONS T TANKED ELECTRICAL APPARATUS Thomas William Hunt, Hoole, Chester, and John Edward Cox, Rainhill, near Liverpool, England, assignors to British Insulated Callenders Cables Limited, London, England Filed May 22, 1961, Ser. No. 111,677 Claims priority, application Great Britain May 26, 1969 14 Qlaims. (Cl. 174-452) This invention relates to a bushing insulator, for making a high voltage connection to electrical apparatus enclosed in a fluid tight tank, comprising a body of insulating material through which a conductor for making the conncction passes and means for supporting the body on the tank and attaching it in a fluid tight manner to the tank wall through which the conductor passes. The body will usually be of a ceramic insulating material but itmay be of any other suitable solid insulating material. Its functions are to support the conductor and to insulate it from the tank Wall, both by providing a suflicient thickness of insulating material between the conductor and the wall and byproviding adequate surface leakage paths between the conductor and the tank wall outside or both inside and outside the tank.

As object of the invention is to provide a bushing insulator for use at very high voltages which has a low inductanceand is appropriately shaped for coupling to other apparatus through low inductance connections.

The bushing insulator in accordance with the invention comprises an insulating body formed with a neck, adapted to pass through the wall of the tank and having a central aperture for the conductor, and a head integral with the neck and preferably having a flat surface on its side opposite to that from which the neck projects. The maxi mum overall dimension of the neck, measured at right angles to its axis, is notsubstantially greater than, and is preferably equal to or less than, half the shortest overall dimension of the head, also measured at right angles to the axis of the neck; that is to say if the neck is cylindrical and the head is circular the diameter of the neck is preferably equal to or less than half the diameter of the head. We prefer to make both the head and the neck of circular cross-section about a common axis which coincides with the axis of the conductor. The insulator body is supported in a metal casing comprising a dish shaped part which receives the head of the insulator body and a neck part which receives the neck of the insulator body, the neck of the casing being suitable for mounting in an aperture in the wall of a tank with which the bushing is to be associated. When the surface of the head is flat, the dish shaped part of the casing is preferably formed with an outwardly projecting flange having a surface in substantially the same plane as that surface.

To obtain minimum inductance, both the head and neck are preferably made as thin as possible consistent with their having the necessary breakdown strength, that is, in the case of the neck, breakdown of the insulating material filling the space between the bushing conductor and the neck of the casing and, in the case of the head, between the dish shaped part of the casing and the conductor (connected to the bushing conductor) lying on or adjacent to the surface of the head. By limiting the thickness in this way the area of the inductance loop is kept to the minimum, as will be explained in greater detail below.

With the aim of further reducing the inductance, the

casing for the insulator body is preferably of non-magnetic material and the neck of the insulator body is kept as short United States Patent 0 "ice as possible, thus enabling a short bushing conductor to be used.

We prefer to seal the insulator body, either to the easing in which it is supported or to a part of the tank to which the casing is attached, by means of a joint of the kind in which a cylindrical body, for example the neck of the insulator body, is made a good fit in a tubular body, for example the neck of the casing, and the joint between them is made by means of one or more yieldable sealing rings each seated in an annular groove in the surface of the neck of the insulator body or in the internal surface of the neck of the casing, the sealing rings being deformed under compression when the neck of the insulator body is forced into the neck of the casing to form a' tight joint. This type of joint, which is the subject of Patents Nos. 2,639,350 and 2,924,644, will hereinafter be referred to as a joint of the kind specified. It is designed to withstand hydraulic or gas pressures which may exist or arise in the tank and hence to resist separation of the insulator body from its casing.

Three forms of bushing insulator and a method of making an electrical connection to a bushing insulator, in accordance with the invention, will hereinafter be described by way of example with reference to the accom panying drawings in which;

FIGURES 1, 2 and 3 are sectional elevations of the three bushing insulators,

FIGURE 4 is a plan of the insulator shown in FIG- URE 3,

FIGURE 5 is a section of a modified detail of the bushing insulator shown in FIGURES 3 and 4, and

FIGURE 6 is an isometric view, partly in section of the electrical connection.

Referring to FIGURE '1, the first insulator comprises an insulator body of ceramic material consisting of a head 1, in the form of a flat disc 6 in diameter and /2 thick, provided with an integral co-axial cylindrical neck 2, 3" in diameter and 1 /2 long. The insulator is designed for mounting on a flat lid 301: a capacitor tank, shown horizontal in the drawing but adapted to lie in any other plane; for example the tank may lie on its side in a rack with the lid in a vertical plane and the axis of the bushing horizontal. The body has an axial bore 4 of average diameter about 1 for receiving the bushing conductor (not shown).

The body is mounted in a metal casing, comprising a dish shaped upper part 5 into which the head 1 fits and a tubular lower part forming a neck 6 into which the neck 2 fits. The dish shaped part 5 is detachable from the neck 6 of the casing. The upper edge of the casing is formed with a flange 7 which is flush with the upper surface of the insulator body, the outer boundary of the flange being a square of side length 7". The flange is provided with holes and slots, as required, for the attachment of electrical connections and/ or for bolting to the flanges of adjacent bushing insulators. The lower end of the neck 6 of the casing terminates flush with the lower surface of the neck 2 of the insulator body. The neck 6 of the casing is a good lit in an aperture in the lid 3 of the capacitor tank and is soldered to the lid, leaving a small clearance between the lower surface of the dish shaped part 5 of the casing and the upper surface of the lid 3.

The outer surface of the neck 2 of the insulator body is formed with a shallow rectangular circumferential groove into which a ring of neoprene 8 of the outside diameter slightly greater than the outside diameter of the neck fits. After the neck 6 of the casing has been attached to the lid 3 the dish shaped part 5 of the casing is secured to it and the neck 2 of the insulator body is forced into the casing, thus making a joint of the kind specified between the neck 2 of the insulator body and the neck 6 of the casing. To facilitate the beginning of this operation, the upper end of the neck 6 is tapered as shown. As the neck 2 of the insulator body is forced into the neck 6, the neoprene ring will be extruded as shown into the annular space between the outer surface of the neck 2 and the inner surface of the neck 6.

Similarly the wall of the central bore 4 in the insulator body is formed with a groove to receive a neoprene ring 9 to enable a joint of the kind specified to be made between the bushing conductor and the insulator body. This conductor can be in the form of an outer tubular part fitted into the bore in the insulator body and adapted to receive a central rod shaped part projecting sufficiently from the upper and lower surfaces of the insulator body to enable electrical connections to be made.

Referring to FIGURE 2, the second insulator is similar to the first except that the neck 10 of the casing is shorter and is provided with an ex ernal screw thread, leaving a short length of the neck ll of the insulator body projecting from the lower end of the neck It) of the easing. An internally threaded sleeve 12 adapted to receive the threaded neck Id of the casing is permanently attached to the lower surface of the tank lid 3 around the aperture. At the lower end of the sleeve 12 there is an inwardly projecting flange 13 of a Width equal to the thickness of the neck It) of the casing. On the upper surface of this flange is a seating for a neoprene ring i i. As in the first described insulator, the neck ill of the insulator body is sealed to the neck Iii of the insulator casing by a joint of the kind specified including a neoprene ring 15, but this joint can be made before the bushing is fitted in the lid 3 of the capacitor tank. The outer part 16 of a two-part bushing conductor is sealed in an axial bore in the insulator by means of a joint of the kind specified incorporating a neoprene ring 1'7.

After the body has thus been fitted into the casing, the casing is screwed into the threaded sleeve 12 in the tank lid 3 until the lower end of the neck ll of the casing bears against the neoprene ring 14 to compress it and form a seal between the threaded sleeve 12 and the neck 10 of the casing. The co-operating threads on the sleeve 12 and the neck Ill of the casing are fine threads, so that the angular position of the bushing can be adjusted while still maintaining sufficient pressure on the ring 14 to form a seal.

Referring to FIGURES 3 and 4, the third form of bushing differs from the first two described in that the insulator body is formed with a longer neck It which projects by about 1" from the end of the neck 19 of the casing and that the neck 18 of the body is not sealed to the neck 19 of the casing by a joint of the kind specified but to a sleeve Zii attached to the tank lid 21.

As in the second bushing described above, the outer surface of the neck 19 of the casing is screw threaded and the sleeve 20, which is permanently attached in the lid 21 of the casing, is internally threaded. In this example, however, the lower end of the sleeve 2d is formed with two internally projecting flanges 22 and 23, between which a neoprene ring 24 of approximately square crosssection fits with its inner edge projecting from the groove formed by the flanges 22 and 23. The inner diameter of the ring is slightly less than the outer diameter of the neck 18 of the insulator body and the extremities of the flanges are a good fit on the neck 18 so that when the casing containing the insulator body is screwed into the threaded sleeve it? in the tank lid 21, the neck I3 of the body is forced into the neoprene ring 24 which makes a joint of the kind specified directly between the sleeve 29 attached to the tank lid and the projecting part of the neck 18 of the insulator body. To facilitate the making of this joint, the extreme lower end of the neck 18 is of smaller diameter than the main part of the neck and the neck is tapered between the parts of different diameter. The arrangement has the advantage that angular adjustment of the bushing can be effected without altering the effectiveness of the seal between the bushing and the tank lid.

When the correct angular adjustment of the bushing has been attained, the bushing can be locked in position by a lock nut 25 carried on the threaded part of the neck 19 of the casing. The threaded sleeve 2t) mounted in the lid 21 projects through the upper surface of the lid to form a narrow seating for the lock nut 25', thus ensuring good electrical connection between the tank lid and the casing of the bushing.

As in the previous examples, the bushing conductor is sealed in a bore in the insulator body by one or more joints of the kind specified. This conductor is in two parts, an outer sleeve 27 of the same length as the length of the bore in the insulator body and an inner rod 23 secured by a nut 2? and projecting from the upper and lower surfaces of the insulator body to provide the necessary electrical connections. The outer sleeve 27 is grooved at 30 and 31 to receive neoprene rings for forming the joints with the insulator body.

The bushing differs from those shown in FIGURES l and 2 in that the head 33 is not a tight fit in the dish shaped part 34 of the casing and that its flat major surface (the upper surface) lies in a plane slightly below the plane of the flange 35 of the casing, a resilient washer 36 being interposed between the head and the casing. This avoids troubles due to inaccuracies in the dimensions of the insulator body.

The flange 35 on the casing is rectangular (FIGURE 4) and is formed with slots 3'7, plain holes 33 for bolted connections, and threaded holes 39 for screwed connections. The slotted edge of the flange is reduced in thickness by cutting away its under surface and the opposite edge of the flange is reduced in thickness by cutting away its upper surface (as shown) so that a lap joint with a flush upper surface can be made between the flanges of adjacent bushing insulators.

With all three bushings described by way of example, the central rod shaped part of the conductor (such as 28 in FIGURE 3) can be connected to the electrical apparatus in the tank, before the insulator is fitted into the aperture. After the insulator has been fitted over this conductor and secured in the desired position, the inner conductor can be connected to the outer conducting sleeve by the nut 29 and sealed, as by soldering (32) in FIG- URE 3.

FIGURE 5 shows an alternative method of sealing an inner conductor 40 to an outer conductor 41, by means of a flexible O-ring 42, that can be used in any of the bushing insulators shown in the drawings. When this form of seal is used it is made by tightening a nut such as 29 (FIGURE 3) and the soldering 32 is not necessary.

FIGURE 6 shows a preferred method of electrically interconnecting bushings in accordance with the invention. A flat conductor 43, which is apertured where it fits over the heads of the insulator bodies, is attached to the flan es (44 and 45) of a number of bushings (46 and 47). It may for example be attached by countersunk screws passing into threaded holes in the flange such as the holes 39 disposed around the insulator head in FIG- URE 4. The apertures in the conductor 43 are filled with discs of insulating foil 4% and a layer of insulation is built up on the flat conductor from foil 49. A second flat conductor 5% is then connected to the bushing conductors 51 and 52. In some cases the use of the first of the two flat conductors (43) can be avoided by bolting the adjacent flanges of bushings together utilising slots 37 and bolt holes 38 as in FIGURE 4.

Similar connecting means to that shown in FIGURE 6 can be used for connecting one of the bushing insulators or a series of interconnected bushing insulators to another terminal arrangement, for example, a common terminal for a bank of capacitors each enclosed in a fluid tight tank provided with a bushing insulator in accordance with the invention.

In all cases the diameter of the fiat surface of the head of the insulator body is made sufficiently long to provide an adequate leakage path between the bushing conductor and the flange on the dish shaped part of the casing and the thicknesses of the head and of the neck of the insulator body are such that failure of the insulating material by puncture will not occur under normal stress conditions.

When the connections have been made as described above, the inductance loop is formed by the central conductor of the bushing, the flat conductor connected thereto, the dish shaped casing for the insulator body and the neck of that casing. This loop is preferably made of minimum area by making the thickness of the head and of the neck of the body the minimum necessary to provide the necessary breakdown strength (through the insulating material) and by making the diameter of the head the minimum necessary to provide the necessary length of surface leakage path between the bushing conductor and the flange on the casing. The insulator also has an inherently low inductance by virtue of the fact that it is co-axial in nature.

What we claim as our invention is:

1. In a high voltage connection to electrical apparatus enclosed in a fluid tight tank, a bushing insulator comprising an insulating body formed with a head integral with a neck, the surface of the head opposite to that from which the neck projects being substantially flat and of dimensions such that the maximum overall dimension of the neck is not substantially greater than half the shortest overall dimension of said surface of the head, both measured at right angles to the axis of the neck, the neck having a central aperture for a conductor and being adapted to pass through the wall of the tank, and a metal casing in which the insulator body is supported comprising a dish shaped part which receives the head to leave only said surface of the head exposed, and a neck part which receives the neck of the insulator body and is mounted in an aperture in the wall of the tank.

2. In a high voltage connection to electrical apparatus enclosed in a fluid tight tank, a bushing insulator comprising an insulating body formed with a circular, discshaped head integral with a co-axial cylindrical neck having a central aperture for the conductor and being adapted to pass through the wall of the tank, the surface of the head opposite to that from which the neck projects being substantially fiat and of a diameter such that the diameter of the neck is not substantially greater than half the diameter of the head, and a metal casing in which the insulator body is supported comprising a dish shaped part which receives the head, to leave only said surface of the head exposed, and a neck part which receives the neck of the insulator body and is mounted in an aperture in the wall of the tank.

3. In a high voltage connection to electrical apparatus enclosed in a fluid tight tank, a bushing insulator comprising an insulating body formed with a head, having a fiat major surface, integral with a neck projecting from the opposite side of the head to the flat surface and perpendicular thereto, the neck, which has a central aperture for the conductor and is adapted to pass through the wall of the tank, being of a maximum overall dimension not substantially greater than half the maximum overall dimension of said flat surface of the head, both dimensions measured at right angles to the axis of the neck, and a metal casing in which the insulator body is supported comprising a dish shaped part which receives the head, a flange with a surface substantially in the plane of the flat surface of the head projecting outwardly from the rim of the dish shaped part and a neck part which receives the neck of the insulator body and is mounted in an aperture in the wall of the tank.

4. In combination a number of tanks incorporating high voltage connections as claimed in claim 3 in which the flange of each bushing insulator is connected to the flange of a similar bushing insulator with those surfaces of the two flanges lying substantially in the plane of the flat major surfaces of the insulator heads in the same plane.

5. In a high voltage connection to electrical apparatus enclosed in a fluid tight tank, a bushing insulator comprising an insulating body formed with a head integral with a neck, the surface of the head opposite to that from which the neck projects being substantially flat and of dimensions such that the maximum overall dimension of the neck is not substantially greater than half the shortest overall dimension of said surface of the head, both measured at right angles to the axis of the neck, the neck having a central aperture for the conductor and being adapted to pass through the wall of the tank, a metal casing in which the insulator body is supported comprising a dish shaped part which receives the head, to leave only said surface of the head exposed, and a neck part which receives the neck of the insulator body and is mounted in an aperture in the wall of the tank, and at least one yieldable sealing ring seated in an annular groove in one of the mating surfaces of the neck of the insulator body and the neck of the casing, the neck of the insulator body being a good fit in the neck of the casing such that said sealing ring is deformed under compression to make a tight joint.

6. In a high voltage connection to electrical apparatus enclosed in a fluid tight tank, a bushing insulator comprising an insulating body formed with a head integral with a neck of a maximum overall dimension not substantially greater than half the shortest overall dimension of the head, both measured at right angles to the axis of the neck, the neck having a central aperture through which the conductor passes and being adapted to pass through the wall of the tank, and a metal casing in which the insulator body is supported comprising a dish shaped part which receives the head and a neck part which receives that part of the neck of the insulator body that is adjacent the head, a sleeve in which the neck of said bushing insulator is mounted passing through an aperture in the wall of the tank, and a yieldable sealing ring seated in an annular groove in a surface of said sleeve contiguous with a surface of the part of the neck of the insulator projecting from the neck of the casing, the neck of the insulator body being a good fit in the grooved part of the sleeve such that said sealing ring is deformed under compression to make a tight joint.

7. A high voltage connection as claimed in claim 1 in which the maximum overall dimension of the neck is less than half the shortest overall dimension of the head, both dimensions being measured at right angles to the neck axis.

8. A high voltage connection as claimed in claim 6 in which the maximum overall dimension of the neck is less than half the shortest overall dimension of the head, both dimensions being measured at right angles to the neck axis.

9. A high voltage connection as claimed in claim 1 in which the casing is of a non-magnetic metal.

10. A high voltage connection as claimed in claim 6 in which the casing is of a non-magnetic metal.

11. A high voltage connection as claimed in claim 1 comprising a tubular conductor which is a good fit in the central aperture in the neck of the insulator body and at least one yieldable sealing ring seated in an annular groove in one of the mating surfaces of the neck and the tubular conductor, said sealing ring being deformed under compression to make a tight joint.

12. A high voltage connection as claimed in claim 6 comprising a tubular conductor with is a good fit in the central aperture in the neck of the insulator body and at least one yieldable sealing ring seated in an annular groove in one of the mating surfaces of the neck and the 3,1 g tubular conductor, said sealing ring being deformed under compression to make a tight joint.

13. A high voltage connection as claimed in claim 6 in which the neck of the casing of the bushing insulator is externally threaded and the sleeve passing through the tank Wall is internally threa ed to receive said neck, and a lock nut running on the thread on said neck bears on the end of the sleeve to lock the bushing insulator in an angular position into which it is adjusted by rotation of the casing with respect to the sleeve.

14. In combination a fluid tight tank and a bushing insulator, for making a high voltage connection to electrical apparatus enclosed in said tank, comprising an insulating body formed with a head integral with a neck, the surface of the head opposite to that from which the neck projects being substantially flat and of dimensions such that the maximum overall dimension of the neck is not substantially greater than half the shortest overall dimension of said surface of the head, both measured at right angles to the axis of the neck, the neck having a 20 central aperture for a conductor and being adapted to pass through the wall of the tank, and a metal casing in which the insulator body supported comprising a dish shaped part which receives the head to leave only said surface of the head exposed, and a neck part which receives the neck of the insulator body and is detachably mounted in an aperture in a wall of the tank in such a way that the bushing insulator is rotatable about the axis of the neck with respect to the tank.

References Cited in the file of this patent UNITED STATES PATENTS 732,032 Barber June 30, 1903 2,640,118 Werner May 26, 1953 2,777,923 Munson et al. Ian. 15, 1957 2,840,262 Learmonth et al June 24, 1958 2,934,668 Dall Apr. 26, 1960 2,968,020 Barnhart Jan. 10, 1961 

1. IN A HIGH VOLTAGE CONNECTION TO ELECTRICAL APPARATUS ENCLOSED IN A FLUID TIGHT TANK, A BUSHING INSULATOR COMPRISING AN INSULATING BODY FORMED WITH A HEAD INTEGRAL WITH A NECK, THE SURFACE OF THE HEAD OPPOSITE TO THAT FROM WHICH THE NECK PROJECTS BEING SUBSTANTIALLY FLAT AND OF DIMENSIONS SUCH THAT THE MAXIMUM OVERALL DIMENSION OF THE NECK IS NOT SUBSTANTIALLY GREATER THAN HALF THE SHORTEST OVERALL DIMENSION OF SAID SURFACE OF THE HEAD, BOTH MEASURED AT RIGHT ANGLES TO THE AXIS OF THE NECK, THE NECK HAVING A CENTRAL APERTURE FOR A CONDUCTOR AND BEING ADAPTED TO PASS THROUGH THE WALL OF THE TANK, AND A METAL CASING IN WHICH THE INSULATOR BODY IS SUPPORTED COMPRISING A DISH SHAPED PART WHICH RECEIVES THE HEAD TO LEAVE ONLY SAID SURFACE OF THE HEAD EXPOSED, AND A NECK PART WHICH RECEIVES THE NECK OF THE INSULATOR BODY AND IS MOUNTED IN AN APERTURE IN THE WALL OF THE TANK. 